1. Field of the Invention
The present invention relates to the use of cold-weld pinch-off devices to close pressure vessels. More specifically, it relates to crimp sealing of tubes flush or below a fixed surface to achieve a leak tight seal.
2. Description of Related Art
A need exists in vacuum and pressure systems to close pressure vessels with a leak tight seal. Metal seals are often preferred over elastomeric seals such as o-rings, due to the high reliability and resistance to hostile environments that can be achieved with metal seals.
One method of closing a vessel (FIG. 1) with a metal seal is to have a small ductile metal tube 2 attached to the vessel 4. FIG. 2 shows how tube 2 may be pinched and severed by a crimping tool 6. This technique offers some advantages over welding or brazing such as reduced risk of exposure to the vessel contents and no need for large amounts of thermal energy input to the vessel. Furthermore, tube crimping is sometimes preferred over using gaskets, flanges, valves and fittings because of the reduced bulk and weight that results.
One disadvantage of crimp sealing tubes is the remnant of the crimped tube itself which is left protruding from the vessel. The tube remnant may be very sharp and pose a hazard to people and it may be delicate and require some type of protection to avoid reopening the vessel.
U.S. Pat. No. 4,727,233 is directed to a method for sealing tubes that includes the steps of locally compressing the portion of the tube to be sealed with a pressure capable of achieving a seal, clamping of the tube, cutting the compressed portion of the tube, welding the lips of the part which is still clamped, and finally removing the damping pressure. U.S. Pat. No. 4,512,488 is directed to a method for sealing oval shaped tubing using crimping with severing. U.S. Pat. No. 4,287,746 discloses a device for crimping and severing capillary tubes using cold welding. U.S. Pat. No. 3,505,556 is directed to a symmetric pinch-off crimp for incandescent lamps with a filament sealed into the crimp. U.S. Pat. No. 3,251,525 is directed to an apparatus for sealing tubulations by pinching off and cold weld sealing the tubulation using pinch jaws.
U.S. Pat. No. 3,334,407 is directed to a method for making a rupturable fluid bearing container by crimping a metal tube with pinch-off dies. U.S. Pat. No. 3,263,465 is directed to an apparatus for severing and sealing a hollow conduit, resulting in a symmetric seal. U.S. Pat. No. 2,776,473 is directed to a method of tube sealing by pinching off and sealing using crimping jaws. U.S. Pat. No. 2,427,597 is directed to a method for cold weld sealing of tubes using symmetric crimp dies. U.S. Pat. No. 3,266,287 discloses an apparatus for closing and reopening a metal tube. Symmetric crimping is employed. U.S. Pat. No. 3,260,098 is directed to a crimping tool for closing and reopening a metal tube.
Conventional tube crimping is typically done with tools that satisfy these requirements:
a. A symmetric pair of dies made of hardened tool steel are needed. Frequently these dies are made to very precise machining tolerances and the die shape is customized for a particular application. PA1 b. A mechanism is needed to guide and align the dies while a very large force concentration is applied. This may be achieved by installing the dies into a block with a screw driven arrangement, or a lever mechanism similar to those used for bolt cutters may be used. PA1 c. Dies must be retractable for disassembly.
For a conventional symmetric tube crimping configuration, the tube is crimped above a fixed surface on a pressure vessel. An equal force is exerted on symmetric dies, both having blunt tips This angle results in large components of shear and tensile stress in sections of the tube which would be in nearly pure compression if a conventional crimping device was used. The shear stress improves the quality of the seal. However, the tensile stress causes stretching in the portion of the tube that is to be removed if the tube is not allowed to move with the die. If the tube stretches, it will have a reduction in cross sectional area which results in a reduction of the amount of compression on the tube.